This invention relates to a loop transmission system in which packet communications are performed between terminals of the loop transmission system including a plurality of transmission/reception terminals and a control terminal which are connected in a loop.
A time division multiplex system utilizing a frame construction has been known as a communication system for a network including coexistent voice terminals and data terminals. This system is suitable for voice because it can assure a real time characteristic but unsuitable for data because it is difficult to readily accommodate data terminals having various speeds and because it can not accommodate high speed data terminals. As another system a packet multiplex system has been proposed in which due consideration is made for data. Although this system can be constructed as a versatile system suitable for various data speed terminals, for voice there is a delay depending upon the activity of the line thus failing to assure a real time characteristic. As an example of a signal having the real time characeristic may be mentioned a moving picture image signal in addition to the voice signal.
As an approach for improving the two systems described above, a system has been proposed in which a frame 100 is divided into two subframes 101 and 102 as shown in FIG. 1, one subframe 101 being used as a time division type subframe for voice and the other subframe 102 being used as a packet multiplexing subframe. This system, however, has a defect that its efficiency decreases when the quantity of the traffic is increased by either one of the voice and data. Thus, for example, where the quantity of traffic of the voice is large while that of the data is small, even when the data subframe is vacant or idle, it is not possible to use it for the voice, thus decreasing the utilization efficiency of the system. To cope with this problem, a system has been proposed in which a partitioning line of the frame 100 is moved according to the traffic condition, as described by B. Maglaris and M. Schwartz in a paper entitled "Performance Evaluation of a Variable Frame Multiplexer for Integrating Switched Networks", IEEE Transactions on Communications, June, 1981, Vol.COM-29, No. 6. The system described in this paper, however, requires a central control terminal that supervises the traffic state so that it is defective in that the control is extremely complicated.
As a method of efficiently transmitting and receiving voice and data signals over a loop shaped transmission line, there is a register inserting method described in E. R. Hafner's paper entitled "A Digital Loop Communication System", IEEE Transactions on Communications, Vol. COM-22, No. 6, June 1974. Each terminal utilizing this register inserting method has a basic construction including a receiving resistor 202, a transmitting register 203 and a switch 204 as shown in FIG. 2. In FIG. 2, the length of the register is equal to the packet length.
A method of control will now be described with reference to FIG. 3 in which sections (a) through (f) show states. The flows of the data on the switch are shown on the left, while the states of the switch are shown on the right. Blocks on the left showing the flow of the data on the switch represent packets and alphabets in the blocks represent packet names. Among the blocks on the right showing the switch state, block 301 designates the receiving register, block 302 the transmitting register and the alphabets in the registers designate the packet names stored therein. The switch is normally thrown to a stationary contact 1 to establish a bypass state. Suppose now that a packet A is passing through the switch and a packet B follows immediately thereafter, and that at this time a terminal requests transmission of a packet D and this request is set in the transmitting register 302. This state corresponds to section (a) in FIG. 3. When the last bit of packet A has passed through, the switch is transferred to a stationary contact 3, as shown at (b) in FIG. 3. Under this state, the transmitting packet D is sent out to the loop as shown at (c) in FIG. 3. When the switch is transferred from contact 3 to contact 2 upon completion of the transmission, the packet B has already been stored in the receiving register without being lost as shown at (d) in FIG. 3. Thereafter, the receiving register will be continuously inserted in the loop as shown at (e) in FIG. 3. If the state of section (e) in FIG. 3 continues for a long time, transmission of a new packet becomes impossible. But if the switch is transferred randomly to terminal 1, the packet now passing through the receiving register 301 will sometimes be lost. The switch may therefore be transferred when only a vacant packet is stored in the receiving register 301. Alternatively, the most simple and accurate method for transfer of the switch may be such that the switch is transferred to terminal 1 when the transmitted packet D circulates through the loop and stored in the receiving register 301. The state of transferring the switch under this state is shown at (f) in FIG. 3. By the control described above, packets pass through transmitting and receiving terminals. This register inserting method is characterized in that transmission of the packet can be made without any appreciable waiting time irrespective whether the loop is busy or not, and that exchange control can be completely dispersed. Further, the data transfer time including a waiting time at a terminal is so short that the throughput characteristic can also be improved. However, the data transfer time of this register insertion method is governed by whether the loop is busy or not so that the data transfer time is indefinite. Accordingly, this method is not suitable for voice communication.